The present invention relates generally to methods and apparatuses for powering undersea optical communications systems, and more particularly, to a method and apparatus for powering an undersea optical communication system employing multiple branching units.
Undersea optical communication systems include land-based terminals containing transmitters and receivers connected by a cabled-glass-transmission medium that includes periodically spaced repeaters, which contain optical amplifiers whose purpose is to compensate for the optical attenuation in the cabled fiber. As the repeaters are usually placed undersea and away from power sources, power must be supplied remotely to the repeaters. The cabled fiber therefore usually contains a copper conductor to carry electrical power to the repeaters from the terminals. These undersea systems serve to carry optical communication signals (i.e., traffic) between the terminals. The traffic on these systems can consist of voice, data, television, Internet traffic, international telephone traffic, etc. Consequently, the revenue lost when the system is down can be significant. Therefore, these systems must have high reliability and availability.
To provide increased flexibility in undersea network architecture beyond simple point-to-point interconnection between land-based terminal, a branching unit is provided, which allows traffic to be split or switched to/from multiple landing points. Conventional branching units typically manage the cabled-fiber interconnections and the power conductor paths among three cables. The latter is necessary to maintain as much traffic carrying capability when a fault occurs in one of the three cable legs, which increases the availability of the system.
The most common type of electrical undersea fault is a shunt, that is, a current leakage path that develops between the power conductor and the seawater without necessarily a break in the power conductor. A shunt fault is often the result of external aggression to the cable from fishing trawlers, ships"" anchors and the like. At the site of a shunt fault, the optical glass fibers are often left intact, but sometimes are damaged as well.
Another common fault resulting from external aggression is a complete cable break, also referred to as a cable cut, where the power conductor is parted and generally exposed to seawater at both ends. Of course, the glass fibers in this case are broken and parted as well. When a shunt or broken cable fault occurs in a system containing branching units, it is often necessary to re-configure the powering in order to:
1) maximize the traffic carrying capability on the faulted portions without faults; and
2) maintain the portion containing the fault in an off-powered state and grounded so a repair can be made safely.
A power-switched branching unit is configured to allow re-routing of electrical power from the terminals in the presence of a fault in one of the cables, so that two of the three cable legs in a branched system can still be powered. Such a power-switched branching unit usually has three operating states: normal, alternate-normal, and grounded-trunk. The power-switched branching unit can be configured in any of these three states by the appropriate power-up sequencing from the terminals of the three legs.
One of the three cables connecting to a branching unit is known as the trunk and the other two as branches. In an existing undersea cable system containing a power-switched branching unit, in a normal powering state, current flows from the terminal connected to the trunk through the branching unit to one of the branches. At the branching unit, the other branch is grounded to enable it to be powered from the terminal to which it connects. Thus, the amount of current in each of the branches is the same as on the trunk. Refer to FIG. 1A, which illustrates this powering configuration. In an undersea communications system, the terminals are typically located on land and the branching units (along with the cable and repeaters) are typically located under water. As shown in FIG. 1A, the power circuitry in the branching units grounds one branch while allowing the current from the trunk to pass through to the other branch. Which branch power path is grounded at the branching unit and which is connected to the trunk power path depends on the sequence of power turn up from the terminals.
If a shunt fault occurs in the trunk or the branch powered via the trunk, the voltage applied from the corresponding terminals can be adjusted to provide a virtual ground (i.e., a point at which the voltage is zero Volts) at the fault site. With powerfeed equipment operated in the constant current mode, movement of the virtual ground to the fault site is automatic. Thus, traffic can be maintained (provided the fibers are not damaged) because no current is lost via the shunt path to ground. However, if the shunt fault occurs in the separately powered branch (which is grounded at the branching unit), there is no capability for placing a virtual ground at the fault site to maintain traffic carried via that branch. It is possible, however, to power down the system and re-power the equipment in the new configuration shown in FIG. 1B, such that a virtual ground can be placed at the fault site and all traffic can be maintained as before. Unfortunately, traffic is lost during the power reconfiguration. Thus, with a power-switched branching unit, it is necessary to power down the system before it can be re-powered in a new configuration. Naturally, all traffic carrying capability is lost when power is removed from the system. When the system contains more than one interconnected branching unit, the power down and power up sequences can be complicated, requiring communications and coordination among the different terminal stations. The necessary communications and coordination result in long power up sequences, during which revenue producing traffic cannot flow. Moreover, traffic may be lost in certain segments of the system that are adjacent to the fault. This loss in traffic occurs because removal of power to the adjacent branching units is required to reconfigure the branching unit with a faulted leg prior to undertaking repair operations, or when returning the branching units to the normal no-fault states upon completion of a repair.
The present invention is therefore directed to the problem of developing a fault-tolerant branching unit that does not require powering down and re-powering to change the power configuration, e.g., to allow a virtual ground to be re-located at shunt fault sites that occur in either trunk or branches. Moreover, the present invention is also directed to the problem of developing a fault-tolerant branching unit that, even with a branch shunt fault or cable break, allows for power to be removed from the faulted branch prior to repair without necessitating powering-down of the other two legs. In addition, the present invention is also directed to the problem of developing a fault-tolerant branching unit that, at the completion of a branch repair, enables restoration of power to the repaired branch without first powering-down the other two legs.
The present invention solves these and other problems by providing that each branch of the system be supplied with a current equal to half the current in the trunk and by coupling the power path of both branches to the trunk so that the current sums at the branching unit. As a result of the above provisions, upon occurrence of a cable fault in any branch, a virtual ground moves to the site of the cable fault, even in the branch with the fault. In addition, an image virtual ground moves to a similar point in the branch that does not have the cable fault.
As a result of this implementation, a system employing the techniques and apparatuses of the present invention is able to tolerate at least one fault in a branch without necessitating down powering and re-powering:
1) to maintain traffic (provided the fault is a shunt and the cabled glass fibers are not damaged);
2) to subsequently allow the branch containing the fault to be powered-down separately to ensure the safety of shipboard personnel during the repair operation while still allowing traffic to be carried over the portions of the system without a fault; and
3) to return to a normal powering configuration upon completion of a repair without affecting traffic being carried over the portion of the system without a fault.